The swiftest and largest predatory fish – tuna, some sharks, billfish like marlins – warm their swimming muscles a few degrees warmer than the surrounding water, and thereby get the extra speed they need to race down prey. Though the rest of their bodies are mostly unwarmed, the same temperature as the water around them, it is a remarkable adaptation.

Tbey do this through a complex heat-exchange tangle of arteries and veins (rete mirabile) near their swimming muscles, conserving the heat instead of losing it all when when the blood flows through the gills where it is cooled to the ocean ambient temperature while it is re-oxygenated. These fish usually also tend to stay near the surface, where it is warmer, dropping down into deeper colder water only to hunt.

Now we know that at least one species, the Opah Lampris guttatus, is endothermic. Not as hot as as mammals and birds are, but surprisingly warmer than the water around them. In 10 degree C water, in a 40 kg fish, muscles and internal organs are about 5 degrees warmer, and the brain and eye muscles a couple of degrees even warmer than that.

Opah also avoid surface waters: they are mesopelagic, living circumglabally at depths 50 to 400 m below the surface. And they are predators, apparently of squid, though they lack the streamlined form we expect of predatory fish.

On the left, internal temperatures, 5 cm below the skin, of a 40 kg Opah in 10.5 degree C water. On the right, temperature of pectoral muscles of a free swimming Opah (red) at depths of 70 meters or more (temp blue, depth black) (science.org)

How do they stay so warm? Their heat is generated by their pectoral muscles, and they too have rete mirabile heat-exchange system, but unlike all other fish, theirs are in their gills, the harshest place they could be. To help conserve their body heat, their skin is unusually thick, and under their skin they have an unusual layer of insulating fat.

The warmer muscles, brain, sense organs and heart all give Opah the advantages of a warm blooded predator in a cold blooded world – more alert, faster. Its body shape is puzzling though – what kind of predatory fish has a body shaped like a disc? Perhaps we don’t know enough about this yet.

What’s ahead for this species in a world where we eat as many fish as we can catch? Though it lives where it is relatively safe from us, it not uncommon as bycatch on the hooks of longline fishers. It also is turning up now in fish markets, and apparently makes for good sushi.

Circumglobal range of Opah (also known as Moonfish). Red denotes regions where it most common. *chefs-resources.com)

But it doesn’t form vulnerable schools, it keeps away from surface waters, its range is very large, and as long as its own food supplies persist, it isn’t severely threatened. Or so it seems, anyway.

It would be nice to understand it better but otherwise, really, let’s just leave it alone, and hope it makes it through these challenging times.